This invention relates generally to a holder for positioning a transparency in a light beam. For instance, such a holder may be used to position a photographic negative between a light source and focusing lens on an enlarger, the negative being used to generate an image on a photosensitive paper for preparing a print or the like. In order to obtain the best possible projected image, it is desireable to maintain the negative flat in a focusing plane for the projection lens, eliminate any distortion from dirt or glass that might occupy the optical path, yet still be able to conveniently adjust the position of the transparency with respect to the optical path.
In the past, transparency holders have sandwiched the transparency between two sheets of glass that were compressed by the weight of a vertically slideable lamp house. Glass was found to distort the transparency image due to optical imperfections, collect dirt and dust, and scratch the delicate transparency surface. In addition, unless the glass was secured to a surrounding frame, it had a tendency to scoot or slide, disaligning the transparency and scratching its surface.
With improvements in the structural characteristics of transparencies, and the increasing popularity of strip film, it became desireable to minimize any contact with the image area of the transparency and limit its supporting contact to the edges thereof. This has been accomplished by a pair of opposed metal plates to grip and maintain the transparency edge. The plates were held in gripping compressed relation by the gravitational weight of the superposed lamp house, and were usually removeable from the projector only by raising the lamp house. Thus, in order to advance a film strip without scratching its surface, it was necessary to lift the lamp house, remove and open the carrier, reposition the transparency strip, close and replace the carrier, and lower the lamp house. If the transparency was misaligned, it was necessary to repeat the operation. Not only was this procedure time consuming and cumbersome, the constant movement of the lamp house imparted relatively severe shocks to a fragile and expensive lamp, reducing its life expectancy. Still further, each time the lamp house was raised or the transparency carrier was removed, light could be emitted by the projector, possibly ruining photosensitive materials.
With still further improvements in transparencies and their image resolution capabilities, a wide variety of film sizes and formats became available for use. However, this necessitated that the user also purchase or obtain a large number of transparency holders or masks that corresponded in size to the transparency projected.